Communication of data between electronic equipment which is separated in physical location is a commmon requirement. In one illustrative application, peripheral equipments are to be attached to a host computer via the adaptor of the instant invention. For short distances, so-called input/output (I/O) channels have been employed for such communication. I/O channels are characteristically fast, and employ many hard-wired conductors and consquently are expensive per foot of distance. Accordingly, they are restricted to short distanced. I/O channels typically utilize simple, fully-interlocked, handshaking protocols to provide control of data flow over the link.
Because of their relatively unsophisticated flow control, many wire conductors are needed to support the handshaking needed to effect the link. For instance, sixteen wires in an Interface Bus manufactured by Hewlett-Packard Co., twenty-two in a communications link described in U.S. Pat. No. 4,276,656 to E. M. Petryck, Jr., and approximately fifty in a MASSBUS manufactured by Digital Equipment Co. Primitive error control is provided by unsophisticated parity checking, for example.
In addition to the problems associated with numerous conductors, four other difficulties are encountered in the use of I/O channels: first, signal integrity is compromised by the degraded signal carried by multiple-conductor buses. The length of the lines accounts for some of this degradation because external noise is induced. Secondly, internal, or cross-coupled energy, induces noise into signals carried on adjacent conducting lines. Thirdly, the parallel trransmission of data produces a timing skew in the signals at the far end of the link. That is, signals may arrive at the receiver in a different order from that in which they were transmitted. Accordingly, a "worst-case" speed penalty is extracted by the need to await complete reception of all signals so that they may be processed by the receive in their proper order. Fourthly, an interactive delay restricting the maximum data rate is imposed by the use of I/O channels, by the propagation delay required to travel the physical distance. For instance, in a four-edge protocol, if a 2.5 microsecond propagation delay is imposed by the physical distance, then 4.times.2.5 or 10 microseconds are needed to conclude parallel transmission of one word on a channel in a four-edge protocol.
In an effort to solve some of the problems associated with I/O channels, so-called channel extenders are known. Instead of many parallel data paths, a signal time-division multiplexed (TDM) serial physical link is employed in conjunction with the channel extenders. The Petryck patent discloses the use of such a system employing a fiber optic physical link. This approach solves the signal integrity problem described above because a single fiber-optic media does not pick up external or internal noise.
However, a new complexity is introduced by the use of channel extenders: namely the need for sampling TDM data transmitted over the single physical link. The Hewlett-Packard Interface Bus designated 37203A or 37203B samples at regular intervals; the system taught by Petryck samples only upon detection of changes in the data pattern to be transmitted. However, in any case problems remain with the use of channel extenders because of the need to physically transmit data in parallel over the (shorter) distance between the "host" electronic equipment and the channel extender which it uses. Furthermore, introduction of a channel extender adds another pair of elements into the data link.
A second approach to implementing a data link between electronic equipment is through so-called data communications technology. Characteristic of this technology is the use of fewer wires than for an I/O channel, thus the ability to economically interconnect over longer distances. Such systems require relatively sophisticated data flow and error control involving highly structured data formats. Typically data is serially communicated in "frames" having various "layers" or levels governed by protocols. Representative of data communication systems are wide-area networks defined by X.25 and HDLC. The latter employing a flow control method called "windowing" which pipelines the serial data communicated over the link. A second type of system is known as local area network (LAN) such as "Ethernet" defined by the IEEE 802.3 standard.
Because the electronic equipment connected to a network using data communication technology must itself transmit and receive data over the system, relatively low transmission rates are realized. For instance, sophisticated error control such as cyclic redundancy checking (CRC) is employed which takes a significant amount of processing by the host. HDLC, for example, provides very clumsy flow control employing, as it does, an address header portion of the frame.
Relatively large blocks of data can be transmitted by data communications technology; each frame including a data block of typically several thousand bits of data. Further benefits of the data communication approach are: first, strictly serial transmission avoids skewing of data, second it is particularly amenable to a fiber optic link because only one physical link is required, and because of the use of highly-structured data packages, sophisticated flow and error control is possible.
As mentioned, however, data communications technology is slow because a dedicated microprocessor is needed to process the data into frames and handle the protocols of the various layers. Furthermore, because data is transmitted in large blocks, a memory external to the microprocessor is needed for temporary storage while the frame with which it is transmitted is composed or processed. Use of such external memories slows the "throughput" of data over the system.
Moreover, because of the use of a general-purpose microprocessor to implement the data communications functions, i.e., algorithms, software programs are executed by the microprocessor which extracts a severe time penalty. Virtually no hardware is dedicated to the data communications function, whereas in I/O channel technology, dedicated hardware is used exclusively allowing simplified and faster transmission.